If you are running a 64-bit machine, make sure that you are installing QGIS from the 64-bit network installer: http://download.osgeo.org/osgeo4w/osgeo4w-setup-x86_64.exe.
I am an OSGeo4W user, for both Windows 64 and 32 bit.
And I must say that the installation made from the network installer seems to me pretty stable. I don't see another reason for the stand-alone installer more than for a Portable GIS.
The standard installation was made only once, and all I do is keep it updated from the advanced mode.
When a tool does not work, 90% of the time is due to a problem with the data or the procedure used.
There is no need to install outdated packages, they will surely not solve the problem and they are even likely to generate some more. When a regression in a package is found, it is reported to the developers and (earlier than later) it is fixed.
These are my current packages for a qgis, grass and saga search in a Win10 64bit machine:
This is the QGIS Settings > Options > System view:
This is the C:\OSGeo4W64\bin\qgis-grass7.bat content:
@echo off
call "%~dp0\o4w_env.bat"
call "%OSGEO4W_ROOT%\apps\grass\grass-7.6.0\etc\env.bat"
call qt5_env.bat
call py3_env.bat
@echo off
path %OSGEO4W_ROOT%\apps\qgis\bin;%OSGEO4W_ROOT%\apps\grass\grass-7.6.0\lib;%OSGEO4W_ROOT%\apps\grass\grass-7.6.0\bin;%PATH%
set QGIS_PREFIX_PATH=%OSGEO4W_ROOT:\=/%/apps/qgis
set GDAL_FILENAME_IS_UTF8=YES
rem Set VSI cache to be used as buffer, see #6448
set VSI_CACHE=TRUE
set VSI_CACHE_SIZE=1000000
set QT_PLUGIN_PATH=%OSGEO4W_ROOT%\apps\qgis\qtplugins;%OSGEO4W_ROOT%\apps\qt5\plugins
start "QGIS" /B "%OSGEO4W_ROOT%\bin\qgis-bin-g7.exe" %*
This is the C:\OSGeo4W64\bin\o4w_env.bat content:
REM Make parent of this script location our current directory,
REM converting UNC path to drive letter if needed
pushd %~dp0
cd ..
REM set OSGEO4W_ROOT to short path version
for %%i in ("%CD%") do set OSGEO4W_ROOT=%%~fsi
REM start with clean path
set path=%OSGEO4W_ROOT%\bin;%WINDIR%\system32;%WINDIR%;%WINDIR%\system32\WBem
for %%f in ("%OSGEO4W_ROOT%\etc\ini\*.bat") do call "%%f"
popd
And this is the C:\OSGeo4W64\bin\qgis-bin-g7.env content:
PATH=C:\OSGEO4~1\apps\qgis\bin;C:\OSGEO4~1\apps\grass\grass-7.6.0\lib;C:\OSGEO4~1\apps\grass\grass-7.6.0\bin;C:\OSGEO4~1\apps\Python37;C:\OSGEO4~1\apps\Python37\Scripts;C:\OSGEO4~1\apps\qt5\bin;{app};C:\Program Files\MiKTeX 2.9\miktex\bin;C:\OSGEO4~1\apps\Python27\Scripts;C:\OSGEO4~1\bin;C:\WINDOWS\system32;C:\WINDOWS;C:\WINDOWS\system32\WBem;C:\Program Files\R\R-3.5.2\bin\x64
GDAL_DATA=C:\OSGEO4~1\share\gdal
GDAL_DRIVER_PATH=C:\OSGEO4~1\bin\gdalplugins
GDAL_FILENAME_IS_UTF8=YES
GEOTIFF_CSV=C:\OSGEO4~1\share\epsg_csv
GISBASE=C:\OSGEO4~1\apps\grass\grass-7.6.0
GRASS_PROJSHARE=C:\OSGEO4~1\share\proj
GRASS_PYTHON=C:\OSGEO4~1\bin\python.exe
JPEGMEM=1000000
OSGEO4W_ROOT=C:\OSGEO4~1
PROJ_LIB=C:\OSGEO4~1\share\proj
PYTHONHOME=C:\OSGEO4~1\apps\Python37
QGIS_PREFIX_PATH=C:/OSGEO4~1/apps/qgis
QT_PLUGIN_PATH=C:\OSGEO4~1\apps\qgis\qtplugins;C:\OSGEO4~1\apps\qt5\plugins
QT_RASTER_CLIP_LIMIT=4096
VSI_CACHE=TRUE
VSI_CACHE_SIZE=1000000
O4W_QT_PREFIX=C:/OSGEO4~1/apps/Qt5
O4W_QT_BINARIES=C:/OSGEO4~1/apps/Qt5/bin
O4W_QT_PLUGINS=C:/OSGEO4~1/apps/Qt5/plugins
O4W_QT_LIBRARIES=C:/OSGEO4~1/apps/Qt5/lib
O4W_QT_TRANSLATIONS=C:/OSGEO4~1/apps/Qt5/translations
O4W_QT_HEADERS=C:/OSGEO4~1/apps/Qt5/include
Reference links
Best Answer
This is an ongoing issue, at least since QGIS 3.6.0. Despite the warning, the values that are produced seem to be correct. Read this thread for a conversation on the topic. https://github.com/qgis/QGIS/issues/29346. It appears that similar warnings can be produced using GDAL through QGIS also.
As I commented above, I have performed the same SAGA calculations using QGIS 3.10.7, 3.14, and 2.12.3. QGIS 2.12.3 does not report any error, QGIS 3.10.7 and 3.14 do. Despite the error, all three versions result in the same values being produced.
Considering this and the conversation in the link provided suggesting that the values are correct and the warning is innocuous, I believe that the warning can be ignored and values can be trusted.